1. Field of the Invention
The present invention relates to the manufacture of aluminum chloride and more particularly to an improved method for accomplishing heating of aluminum chloride in a distillation/purification process.
Description of the Prior Art
The purification of aluminum chloride has long been a perplexing problem, so difficult that current aluminum chloride manufacture avoids the problem by using pure alumina from the Bayer process with low levels of iron, titanium and silicon impurities as the source of aluminum for the manufacture of aluminum chloride. This is, of course, more expensive than direct chlorination of bauxite or other aluminous ores since the processing of the bauxite through dissolution, precipitation and drying steps is highly energy intensive. In addition to the economic disadvantage, there is the growing difficulty of obtaining low silica bauxite as the high grade bauxites are becoming exhausted or more difficult to obtain. Purification by partial condensation of the chlorination process vapors has been shown repeatedly to be an inadequate purification method not only for the separation of iron chloride but also for the separation of silicon and titanium chlorides, particularly titanium which is more difficult to separate and has a more deleterious effect on the final aluminum metal.
Purification by absorption to other high boiling liquids has also been proposed but such systems are complex and add expensive stages to the manufacturing process. Even more undesirable is the problem of reboiling the aluminum chloride from the absorbing liquids. Because of the corrosive nature of aluminum chloride melts the problem of heat transfer through a surface to the still emerges.
Although there are many materials of construction, the constraints of corrosion and high temperature and the need for high heat conductivity limit the choice of materials for heat transfer severely.
The design of an aluminum chloride still divides into the problem of heat transfer to and from the aluminum chloride and the problem of gas-liquid contact. The latter problem can be handled by routine still techniques using ceramic or glass packing and allowing the gas liquid contact to produce the stage wise or differential packed bed contact as desired. The heat transfer on the cooling or condensation side provides no difficulty provided the surface is not over cooled in the condensation stage. On the other hand, heat transfer to provide boil up of aluminum chloride does cause difficulty because of the corrosive nature of the aluminum chloride.
Although the heat could be transferred through a ceramic surface, extensive surface such as needed for such large scale heat transfer becomes expensive and impractical. Similarly, carbon heat transfer surfaces are also undesirable because of the intercalation compounds which are made between iron and aluminum chloride and graphite.
Unfortunately, aluminum chloride and associated covalent impurity chlorides are not satisfactory conductors of electricity so resistance heating, although highly desirable, is not practical in a conventional system.